1. Technical Field
The invention relates to finned grids for nuclear fuel assemblies having a bundle of fuel rods, the grids being of the type comprising several successive beds each formed of parallel plates, the plates of one bed being crossed with those of the following bed so as to define passages for holding the rods in position at the nodal points of a regular lattice and all plates of the grid being fixed to a same frame.
2. Prior Art
Grids of the above-defined type are already known (EP-A-0 239 441) in which all plates having the same orientation are grouped together in the same bed. With this approach, it is possible to hold the rods axially in position provided that some at least of the plates are provided with resilient means exerting on the rods a transverse force applying them against another plate of the same bed. The fins cause mixing of the fluid streams and so a stirring effect which tends to homogenize the temperature of the coolant and so give to the grid a thermohydraulic function.
Fuel assemblies have further already been proposed in which the rods are supported, i.e. held axially in position, by only some of the grids, the others only having a mixing and/or transverse holding function. Publication EP-A-0 187 578 describes such an assembly, having at least one uppermost rod supporting grid and other grids, placed at a lower level, having only a spacer function. Assemblies have also been proposed comprising grids with essentially a thermohydraulic function interposed between supporting grids, in the upper part of the assembly, i.e. in the part where the coolant is at the highest temperature.